Night vision devices are well known in the art. Generally, these devices include an objective lens, image intensifier tube and an eyepiece or ocular lens assembly. There are various types of night vision devices. In one type, the ocular lens assembly may be replaced by a relay lens so as to enable photographic recording of images. In another type, the standard objective lens may be replaced with a larger objective lens to enable use of the device as a binocular viewer, long range observation system or weaponsight. In addition, the device may be used as a remote viewer by utilizing a video sensor in conjunction with the image intensifier tube. Some of these devices provide reticle patterns that are useful for military applications such as range estimating and aiming of weapon fire. In regard to weaponsights, reference is made to copending U.S. patent application Ser. No. 07/647,544 entitled NIGHT SIGHT FOR MISSILE LAUNCHER by Phillips et al., filed on Jan. 29, 1991 and Ser. No. 07/785,048 entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earle N. Phillips, filed on Oct. 30, 1991 and assigned to ITT Corporation, the assignee herein.
Other night vision devices are configured as night vision goggle systems. These systems are used at night by individuals to enhance their ability to perform tasks such as walking, driving, observation, map reading, and others. One type of system is the single tube night vision goggle system which is presently fabricated by various manufacturers. In this regard, reference is made to U.S. Pat. No. 3,781,560 entitled NIGHT VIEWING SYSTEM FOR READING WITHOUT LIGHT which issued to DeBurgh et. al. on Dec. 25, 1973, U.S. Pat. No. 4,266,129 entitled DEVICE FOR VIEWING RESIDUAL LIGHT IN THE VISIBLE AND THE NEAR INFRARED SPECTRUM which issued to Versteeg et. al. on May 5, 1981 and assigned to N.V. Optische Industrie "De Oude Delft", Delft, Netherlands, U.S. Pat. No. 4,392,710 entitled OPTICAL APPARATUS which issued to Rogers on Jul. 12, 1983 and assigned to Pilkington P.E. Limited, England and U.S. Pat. No. 4,463,252 entitled NIGHT VISION GOGGLE SYSTEM which issued to Brennan et al. on Jul. 31, 1984 and assigned to Baird Corporation.
Two types of night vision goggle systems are the AN/PVS-7A and AN/PVS-7B. These systems include an objective lens assembly, image intensifier assembly and an ocular lens system. The ocular lens system further includes a collimator lens assembly and dual relay lens assembly. The collimator lens assembly is positioned to provide a collimated output image of the image intensifier output screen. The relay lens assembly then relays the collimated image from the collimator lens assembly to the user's eye for viewing. Commercial versions of these systems have been converted into night vision binoculars by the addition of longer focal length objective lenses to provide greater magnification. In addition, ITT Electro Optical Products Division of ITT Corporation in Roanoke, Va. has manufactured night vision binoculars for the US military having 3X and 4.5X magnification. Many of these devices utilize GEN II and GEN III image intensifier tubes, which are well known in the industry.
Many manufacturers provide reticle patterns with these prior art systems. Reticle patterns are useful in many military operations. Such operations include forward observer and fire control missions, where reticle patterns are utilized in order to provide accurate range estimating and aiming of weapon fire. As is known in the art, it is desirable that the reticle pattern be positioned at an image plane so that it is in focus to the user. However, this has not been achieved by manufacturers of such systems.
In most single tube systems, including the AN/PVS-7A and AN/PVS-7B, a fiber optic element and a collimator lens are provided. In regard to fiber optic elements, reference is made to U.S. Pat. No. 5,029,963 entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER which issued to Naselli et al. on Jul. 9, 1991 and assigned to ITT Corporation. Typically, the fiber optic element in such systems is recessed inside the image intensifier housing. The fibers of the fiber optic element are formed to provide an output surface having a radius of curvature for the correction of field curvature or other distortion of an image plane. The radius of curvature is matched to the collimator lenses and reverses the expected field curvature of the output image of the collimator lenses. As is known in the art, the image plane is defined by the fiber optic element. Consequently, it is difficult to affix a reticle pattern on the curved output surface of the fiber optic element utilizing techniques known in the art.
Such systems further include a relay lens assembly having a fold mirror. The relay lens assembly serves to reinvert the image produced by the tube in order to provide upright viewing for the user. The relay lens assembly forms an image plane in the middle of the relay lens. This image plane is located at the position where the fold mirror bisects the image at an angle (typically 45 degrees), making the image plane inaccessible for placement of a reticle pattern.
Various methods have been utilized by the prior art to provide a reticle pattern in such systems. A method is to optically project a reticle image through the objective lens assembly and into the input of the image intensifier assembly. In this method, the reticle pattern is not positioned on an image plane. This results in an indistinct and blurred reticle pattern since it is not in focus. In addition, this method introduces undesirable stray light into the objective lens.
Another technique includes etching a reticle pattern on a curved glass substrate. This substrate is then optically bonded onto the curved output surface of the image intensifier fiber optic element. However, this approach has disadvantages. A disadvantage is that spherical aberration occurs due to the thickness of the glass substrate. In addition, the cost of the curved glass reticle is high. Furthermore, it is difficult to remove the bonded reticle without damaging the image intensifier tube.
Moreover, in order to provide maximum effectiveness on very dark nights, it is desirable that the reticle pattern be illuminated so as to provide contrast against dark backgrounds. In addition, it would be advantageous to have the reticle pattern illuminated in a color which contrasts with the green phosphor output provided by the image intensifier tube. It is also desirable that the intensity of the illumination be adjustable for optimum contrast under varying night conditions. As can be ascertained, a reticle pattern generally cannot be displayed in a contrasting color by utilizing the previously described method of optically projecting the reticle pattern as utilized by the prior art.